1. Field of the Invention
The invention pertains to the field of electronic reproduction technology and relates to a method for correcting skewed recording of a printing original on a recording material, in particular, on a printing plate, in an exposer for recording printing originals.
In reproduction technology, printing originals for printed pages that contain all the elements to be printed such as texts, graphics and images are produced. For color printing, a separate printing original is produced for each printing ink and contains all the elements that are printed in the respective color. For four-color printing, these are the printing inks cyan, magenta, yellow, and black (CMYK). The printing originals separated in accordance with printing inks are also referred to as color separations. The printing originals are generally screened and, by using an exposer, are exposed onto films, with which printing plates for printing large editions are, then, produced. Alternatively, the printing originals can also be exposed directly onto printing plates in special exposure devices or they are transferred directly as digital data to a digital printing press. There, the printing-original data is, then, exposed onto printing plates, for example, with an exposing unit integrated into the printing press, before the printing of the edition begins immediately thereafter.
According to the current prior art, the printing originals are reproduced electronically. In such a case, the images are scanned in a color scanner and stored in the form of digital data. Texts are generated with text processing programs and graphics with drawing programs. Using a layout program, the image, text, and graphic elements are assembled to form a printed page. Following the separation into the printing inks, the printing originals are, then, present in digital form. The data formats largely used nowadays to describe the printing originals are the page description languages PostScript and PDF (portable document format). In a first step, the PostScript or PDF data is converted in a raster image processor (RIP) into color separation values for the CMYK color separations before the recording of the printing originals. In the process, for each image point, four color separation values are produced as tonal values in the value range from 0 to 100%. The color separation values are a measure of the color densities with which the four printing inks cyan, magenta, yellow, and black have to be printed on the printing material. In special cases, in which printing is carried out with more than four colors (decorative colors), each image point is described by as many color separation values as there are printing inks. The color separation values can be stored, for example, as a data value with 8 bits for each image point and printing ink, with which the value range from 0% to 100% is subdivided into 256 tonal value steps.
The data from a plurality of printed pages is assembled together with the data of further elements, such as register crosses, cut marks, and folding marks and print control fields, to form printing originals for a printed sheet. This printed sheet data is, likewise, provided as color separation values (CMYK).
Different tonal values of a color separation to be reproduced may be reproduced in the print only by surface modulation of the printing inks applied, that is to say, by screening. The surface modulation of the printing inks can be carried out, for example, in accordance with a dot screening method, in which the various tonal value steps of the color separation data are converted into halftone dots of different size, which are disposed in a regular pattern with periodically repeating halftone cells. During the recording of the color separations on a printing plate, the halftone dots in the individual halftone cells are assembled from exposure points that are an order of magnitude smaller than the halftone dots. A typical resolution of the exposure points is, for example, 1000 exposure points per centimeter, that is to say, an exposure point has the dimensions 10 μm×10 μm. Conversion of the color separation values into halftone dots takes place in a second step during the further processing of the color separation data in the raster image processor. As a result, the color separation data is converted into high-resolution binary values with only two lightness values (exposed or not exposed) that form the pattern of the modulated dot grid. As such, the printing original data of each color separation is described in the form of a high-resolution halftone bitmap that, for each of the exposure points on the printed area, contains a bit that indicates whether this exposure point is to be exposed or not.
In the recording devices that are used in electronic reproduction technology for the exposure of printing originals and printing forms, for example, a laser beam is generated by a laser diode, shaped by optical measures and focused on to the recording material and deflected over the recording material point-by-point and line-by-line by a deflection system. There are also recording devices that, to increase the exposure speed, produce a bundle of laser beams, for example, with a separate laser diode for each laser beam, and expose a plurality of recording lines of the printing form simultaneously each time they sweep across the recording material. The printing forms can be exposed onto the film material so that what are referred to as color separation films are produced, which are, then, used for the production of printing plates by a photographic copying process. Instead, the printing plates, themselves, can also be exposed in a plate exposer or directly in a digital press, into which a unit for exposing plates is integrated. The recording material can be located on a drum (external drum exposer), in a cylindrical hollow (internal drum exposer), or on a flat surface (flatbed exposer).
Flatbed exposers operate for the most part with a rapidly rotating polygonal mirror, whose mirror surfaces deflect the laser beam transversely over the recording material, while, at the same time, the recording material is moved at right angles to the deflection direction of the laser beam. As such, exposure is carried out recording line by recording line. Because, during the movement of the laser beam over the recording material, the length of the light path changes, complicated imaging optics that compensate for the change in size of the exposure point caused by this are needed.
In the case of an internal drum exposer, the material to be exposed is mounted on the inner surface of a partly open hollow cylinder and exposed with a laser beam that is aimed along the cylinder axis onto a deflection device that reflects the laser beam perpendicularly onto the material. The deflection device, a prism, or a mirror, rotates at high speed during operation and, and the same time, is moved in the direction of the cylinder axis so that the deflected laser beam describes circular or helical recording lines on the material.
In the case of an external drum exposer, the material to be exposed, in the form of films or printing plates, is mounted on a rotatably mounted drum. As the drum rotates, an exposure head is moved axially along the drum at a relatively short distance. The exposure head focuses one or more laser beams onto the drum surface, sweeping over the drum surface in the form of a narrow helix. As such, during each drum revolution, one or more recording lines are exposed onto the recording material.
To shorten the exposure time and, therefore, to increase the economy of the exposer, external drum exposers are, preferably, operated with a bundle of N laser beams that, by exposure optics, image a linear array of exposure points on the surface of the recording material, oriented in the axial direction of the exposure drum. The number of laser beams is, for example, N=64, but can also be a multiple thereof. If the exposure drum rotates, N recording lines are, then, exposed in parallel, winding helically around the surface of the exposure drum. The feed speed of the exposure head is set such that, after one drum revolution, it has moved in the axial direction of the drum by a distance that corresponds to the width of the N recording lines. As a result, the N recording lines to be exposed during the next drum revolution immediately follow the N recording lines exposed during the preceding drum revolution. In another operating mode, what is referred to as the interleave writing method, the laser beams are not imaged as N exposure points with a spacing of one recording line width in each case but with a greater spacing that corresponds to the width of a plurality of recording lines. The feed speed of the exposure head is, then, set such that, during successive drum revolutions, the gaps between the recording lines initially exposed are gradually filled with further recording lines. In every case, however, the feed distance per drum revolution is so high that the recording lines are recorded in a noticeably skewed manner in accordance with the helix. As a result, the printing original data in the form of a rectangular raster bit map is distorted to form a parallelogram.
Various methods are known with which the skewed recording of the printing original data in an external drum exposer is compensated. In U.S. Pat. No. 4,591,880 to Mitsuka, a method is described in which the starting point of the recording lines is advanced somewhat from revolution to revolution so that a rectangular matrix of image points is recorded but, overall, in relation to the edges of the recording material, is rotated by the angle of the skewed recording.
In the method disclosed in European Patent Application 1318660 A2, U.S. Patent Publication No. 2003/0106448 to Uemura, in addition to offsetting the starting point of the recording lines from revolution to revolution, the recording material is clamped onto the exposure drum in a skewed manner, to compensate for the angle of the skewed recording so that the image point matrix is exposed rectangularly and parallel to the edges of the recording material.
According to the method described in British Patent No. GB 2129650 A, the correction to the skewed recording is carried out by moving the exposure head on the exposure drum along a line that is inclined with respect to the drum axis by the angle of the skewed recording.
In European Patent Application 0918255 A2, corresponding to U.S. Pat. No. 6,081,316 to Okamura et al., a method is described with which the printing original data is pre-distorted to compensate for the skewed recording. For this purpose, the printing original data is shifted in a buffer memory in the opposite direction to the angle of the skewed recording so that the image point matrix is deformed into a parallelogram inclined in the opposite direction. As a result of the subsequent skewed recording, this pre-distortion is canceled again, and the image point matrix is recorded rectangularly.